1. Field of the Invention
This invention relates in general to subsea sealing systems and, in particular, to a metal sealing system for subsea trees and tree caps and a method to operate the same.
2. Brief Description of Related Art
The working environment for subsea equipment is increasingly demanding as drilling at deeper subsea locations subjects the subsea equipment to higher temperature extremes, higher working fluid pressures, and chemical attack. Current subsea equipment, such as subsea trees used to complete and produce from a subsea well, typically use elastomeric seals to seal bores of the subsea equipment and to seal between components coupling different interacting subsea equipment together. In situations where the subsea tree is capped, for example with a subsea tree cap, elastomeric seals are generally used to seal the subsea tree cap to a head of the subsea tree to prevent seepage of wellbore fluids into the surrounding subsea environment. Unfortunately, elastomeric seals may not have sufficient durability or resiliency to withstand the temperature and pressure ranges as well as the fluid toxicity found in deeper well installations. Thus, the elastomeric seals may not be sufficiently reliable for use during the required lifespan of the subsea tree cap.
To overcome the limitations of elastomeric seals, some tree caps use metal sealing systems to create the seal between the tree cap and the subsea tree. The metal seal systems may provide a seal that will withstand the temperature, pressure, and fluid toxicity issues encountered in deeper well installations. Metal seals are placed in areas to be sealed and energized to seal opposing surfaces. Typically, energizing a metal seal requires significant sealing stresses at the contact areas between the seal and the opposing surfaces to create a gas and fluid tight seal. This may be true even at lower fluid pressures. To create the sealing stresses at the contact areas, a high degree of interference fit, i.e. sufficient overlap between the width of the seal and the width of the annulus, between the seal and the annulus is required. The high degree of interference fit requires a significant external load to fit the seal, typically applied with the static weight of the part being sealed. However, a subsea tree cap does not have sufficient mass to overcome the high degree of interference fit with static weight alone, this necessitates use of a device having sufficient force generating capability to energize the seal. Alternatively, metal sealing systems may use a secondary mechanical device which generates an internal load to push fit the seal into sealing contact with the annulus. In another alternative system, the tree cap includes a secondary mechanical device that creates sealing stresses after the seal is positioned in the annular space.
The difficulty in latching different equipment together subsea to generate a reaction load makes mechanically aided push fit of an interference fit seal problematic for metal seals. Typically where insufficient static weight is available to set a metal seal, the seal is not interference fit; instead, the seal is energized once positioned in the annular space. In some designs, U-shaped metal seals are typically energized using a hydraulic tool capable of generating large forces that drive an energizing ring between legs of the U-shaped seal, thus driving the legs of the U-shaped seal radially outward into sealing engagement with opposing surfaces. These tools add significant weight to the assembly, require very tight tolerancing on parts and may be complex; Consequently, these tools have an inherent risk of failure that comes with that complexity. U-shaped seals are pressure containing seals that are generally formed of high strength material and require significant sealing stresses to function. In addition, for small tree bores of less than five inches, U-shaped seals are generally unreliable and difficult to make. Therefore, the U-shaped seal systems may be unsuitable for use with subsea trees, specifically those with bores of less than five inches. Still further, the U-shaped seal systems require sealing surfaces on both the seal and the opposing surfaces to be in excellent condition for the seal to function. In particular, the seal and the subsea tree must have a good surface finish having no scratches, no defects, and no inclusions.
Subsea tree caps are often run subsea using remote operated vehicles (ROVs). ROVs are typically limited regarding the weight of the articles the ROV can handle. This weight limit renders many of the energizing mechanisms previously described impractical. Therefore, there is a need for a metal sealing system for sealing a tree cap to a subsea tree that is sufficiently simple yet still generates the appropriate stresses or forces to seal between the subsea tree cap and the subsea tree that may be deployed by an ROV.